Mechanisms of adjustment by different pearl millet plant types to varying plant population densities

Pearl millet (Pennisetum glaucum (L.) R. Br.) is commonly grown at a wide r ange of plant population densities, both by design and as a consequence of stand establishment problems. High tillering genotypes are known to compens ate effectively for lower plant population densities through their tillerin g capacity; less is known about the ability of naturally low tillering geno types to adjust to low plant population densities. This is a particular con cern in the case of the Iniadi landrace materials which are currently widel y used in breeding programmes in both India and Africa. This research was d one to determine how effectively the low tillering Iniadi types adjust to l ow plant population densities and how their mechanism(s) of adjustment comp ares to those of higher tillering materials. Two high and two low tillering genotypes were grown over a period of 5 years at plant population densitie s ranging from 12 to 2 plants/m(2), under both high and low fertility regim es at the ICRISAT Centre, India. Both the high and low tillering types adju sted equally well to the reduced plant population densities, as judged by g rain yield, but differed in their mechanism of adjustment. The high tilleri ng, genotypes adjusted, as expected, primarily by increasing productive til ler numbers, with only small changes in individual tiller productivity. The Iniadi genotypes increased productive tiller numbers in response to decrea sing plant population densities to a limited degree, but increased panicle productivity to a much greater degree than the high tillering types. There was no differential effect on adjustment ability between the two types as a consequence of increased fertility, despite the stimulating effect of fert ility on productive tiller numbers. The results are discussed in terms of g eneralized mechanisms of response to changing individual plant environmenta l resources (fertility and space), and in terms of the use of Iniadi germpl asm in pearl millet breeding programmes.